Many wind energy installations are not stand alone installations provided for a single user but feed the generated electrical power into a grid which distributes the power to a large number of different users. As the power consuming devices of the users require electrical power within a certain parameter range, e.g., a specified nominal voltage and a specified nominal grid frequency, measures are necessary to ensure the stability of these grid parameters.
The grid parameters depend on a balance between the power fed into the grid by electrical power generating installations and the power consumed by the consumers. If the produced electrical power fed to the grid is less than the consumed power, the grid frequency drops. On the other hand, if the consumed power is less than the power fed into the grid, the grid frequency increases which could lead to malfunctions of the consumer's electrical devices.
As long as the fraction of electrical power generated by wind energy installations was relatively small compared to the fraction of electrical power generated by other power generating installations, it was not necessary for wind farms to react to variations in the grid frequency. However, as the fraction of electrical power fed into electrical grids by wind energy installations increases it becomes more and more important that these installations are capable of reacting to variations of the grid frequency.
EP 1 282 774 B1 describes controlling the output power of a wind energy generating plant such that it is constant as long as the grid frequency lies in the range between the nominal grid frequency and a threshold frequency which is three per mill higher than the nominal grid frequency. When the grid frequency increases by more than three per mill above the nominal frequency the output power of the plant is continuously reduced.
EP 1 467 463 A1 describes a wind farm and a method of operating the same. The wind farm is driven at a fixed power output, e.g., maximum total power output, while the grid frequency stays within a predetermined range. If, however, the grid frequency exceeds an upper boundary value of said range, the power output of the wind farm is reduced. If, on the other hand, the grid frequency undershoots a lower boundary value of the predetermined range, the power output is increased.
However, the threshold values defined in the prior art are somehow arbitrarily chosen and could lead to difficulties in reacting to deviations of the grid frequency if the grid frequency changes rather quickly.